1. Field of the Invention
In one aspect, this invention relates to alkylation of isobutane with pentenes. In another aspect, this invention relates to a process to react a feedstock comprising isobutane with pentenes in the presence of sulfuric acid catalyst to produce a high octane alkylate as well as a higher octane isopentane gasoline blending component. In still another aspect, this invention relates to a method to reduce sulfuric acid consumption during alkylation.
2. Background
Federal and state clean air regulations related to motor vehicle fuels have imposed a requirement for reduced olefinic content of gasoline. Olefins in gasolines are considered by many to be major ozone precursors. Also, certain regulations severely limit the amount of pentenes (or amylenes) that can be directly blended into gasoline because of their relatively high Bromine number and contribution to higher Reid Vapor Pressure.
With these new clean air regulations, refiners are directing attention to removing pentenes from the gasoline pool.
For over fifty years, mixed olefinic feedstreams have been reacted with an isobutane feed to produce alkylate. These mixed olefinic feedstreams comprise propylene, butenes, and pentenes. Although C.sub.4 olefins (mixed butenes) have been the principal component of mixed olefinic feedstocks fed to alkylation units, a typical alkylation olefin feed stream comprises both C.sub.3 olefins and C.sub.5 olefins (mixed pentenes).
The mixed butenes include isobutylene, 1-butene, trans-2-butene, and cis-2-butene. The C.sub.4 and lower portion of the feed stream also contains diolefinic contaminants such as 1,3-butadiene. The mixed pentenes typically include 1-pentene, trans-2-pentene, cis-2-pentene, 2-methylbutene-1, 2-methylbutene-2, 3-methylbutene-1 and cyclopentene. The C.sub.5 and higher portion of the feed stream also contains diolefinic contaminants such as 2-methyl-1, 3-butadiene, 3-methyl-1,2-butadiene, 1,3-pentadiene and cyclopentadiene.
It has been reported that pentenes, when alkylated, produce a complex alkylate with many products, due in part to the greater number of olefins possible in the pentene system as compared to the propylene or butene system. For the pentene system, it is also reported that the primary alkylation products are more inclined to undergo rearrangement and cleavage than those from lighter olefins and that hydrogen transfer occurs more readily than with lower olefins. It is also known in the art that C.sub.5 feeds typically contain more diolefins than C.sub.4 olefin feeds, and that the C.sub.5 feeds should be selectively hydrogenated to reduce acid consumption during alkylation that is otherwise caused by competing reactions which result from the presence of the diolefins.
Since pentenes have long been a portion of alkylation unit feed, increasing the pentene portion of such feed is one approach to removing pentenes from the gasoline pool.
Certain procedures for alkylating streams containing C.sub.5 olefins and the quality of the alkylate made from such streams are discussed in the prior art as, for example, in the article entitled "H.sub.2 SO.sub.4 Alkylation Shows Promise for Upgrading Gasoline Pentenes" published in the Feb. 17, 1992 edition of the Oil & Gas Journal at pages 72 to 74.
The prior art, as confirmed by such article, has recognized no benefits or penalties associated with combined or separate alkylation of the C.sub.3, C.sub.4, and C.sub.5 feed materials. The prior art thus teaches that there is no specific advantage to processing C.sub.5 olefins separately and teaches that refiners should co-process C.sub.5 olefins along with C.sub.4 and C.sub.3 olefins.